1. Field of the Invention
The present invention relates generally to an apparatus and method for transmitting and receiving data in a CDMA mobile communication system, and in particular, to a data transmission/reception apparatus and method for adjusting reliabilities of data bits mapped to a modulated symbol before transmission.
2. Description of the Related Art
In a communication system, transmission signals are invariably mixed with some kind of distortion and noises. A mobile communication system transmitting and receiving signals via a wireless network is particularly more susceptible to the distortion and noises than is a wired communication system.
For this reason, various methods have been proposed to reduce the influence of the distortion and noises on the mobile communication system. For example, in order to reduce a bit error rate from 10−2 to 10−3 in an Additive White Gaussian Noise (“AWGN”) environment using the typical modulation technique and coding technique, a low signal-to-noise ratio (“SNR”) of about 1 dB to 2 dB is required. On the other hand, in order to obtain the same results in a multipath fading environment, it is necessary to increase the SNR to about 10 dB. However, a method of increasing transmission power in order to increase the SNR for a reduction in the bit error rate may decrease the entire system performance undesirably. Therefore, a technique for effectively reducing or removing the influence of fading, i.e., the influence of distortion or noises without additional power or a loss of bandwidth in both a User Equipment (“UE”) and a Node B is very important to the mobile communication system. One of the effective plans used for this is a channel interleaving technology combined with an error control coding technique.
The interleaving technology interleaves transmission bits before transmission to disperse a portion of data bits, which may be possibly damaged, to several places instead of concentrating the portion on a single place. That is, the interleaving technology prevents a burst error by allowing adjacent bits to be randomly affected by fading.
Meanwhile, codes used for the error control coding technique are divided into a memoryless code and a memory code. The memoryless code includes a linear block code, while the memory code includes a convolutional code and a turbo code. Further, a device for performing coding by the error control coding technique is called a “channel encoder”.
A future mobile communication system requires reliable transmission of high-speed multimedia data, and therefore, a more powerful channel coding technique is needed. A channel coding technique using the turbo code shows performance nearest to the Shannon limit in the light of the bit error rate (“BER”) even in the low SNR. An output of a channel encoder using the turbo code can be divided into systematic bits and parity bits. Here, the “systematic bits” refer to actual signals to be transmitted, while the “parity bits” refer to signals added to help a receiver correct a possible transmission error. However, even the error control coded signals cannot overcome a possible burst error occurring in the systematic bits or the parity bits. Such a phenomenon often occurs while the data pass through a fading channel. The interleaving is one of the techniques for preventing this phenomenon. This interleaving technique prevents generation of the burst error, contributing to an improvement in the channel coding effect.
The interleaved signals are mapped on a symbol-by-symbol basis in a digital modulator. Here, an increase in the order of the modulator results in an increase in the number of bits included in one symbol. Particularly, in the case of a high-order modulation technique of over 8-ary Phase Shift Keying (“8PSK”), one symbol includes 3 or more information bits, and the bits can be classified according to their reliabilities. Here, as to the reliability, in a process of modulating one symbol by the transmitter, the symbol expressing two bits in a macro region like the left/right quadrants or upper/lower quadrants on the X/Y-axis as shown in FIGS. 1 and 2 is said to have “higher reliability”, and the symbol expressing two bits in a micro region is said to have “lower reliability”.
FIG. 1 illustrates a signal constellation diagram for 16-ary Quadrature Amplitude Modulation (“16QAM”) modulation, and FIG. 2 illustrates a signal constellation diagram for 64-ary Quadrature Amplitude Modulation (“64QAM”) modulation.
Referring to FIG. 1, 16QAM-modulated symbols are each comprised of 4 bits, and have a reliability pattern [H,H,L,L], where H represents a bit position having higher reliability and L represents a bit position having lower reliability. That is, the leading two bits have higher reliability and the following two bits have lower reliability.
Referring to FIG. 2, 64QAM-modulated symbols are each comprised of 6 bits, and have a reliability pattern [H,H,M,M,L,L], where H represents a bit position having higher reliability, M represents a bit position having medium reliability and L represents a bit position having lower reliability.
A transmitter for a common High Speed Downlink Packet Access (“HSDPA”) mobile communication system is comprised of a channel encoder, an interleaver and a modulator, as illustrated in FIG. 3.
Referring to FIG. 3, N transport blocks are provided to a tail bit generator 310, where tail bits are added to the N transport blocks each. A channel encoder 312 encodes the bits constituting each of the N tail bit-added transport blocks, and outputs coded bits. The channel encoder 312 has at least one coding rate in order to encode the N transport blocks. The coding rate can be 1/2 or 3/4. The channel encoder 312 can obtain a desired coding rate through code symbol puncturing or symbol repetition using an R=1/6 or 1/5 mother code. Further, when supporting a plurality of coding rates, the channel encoder 312 is required to select a coding rate to be used among the supportable coding rates by controlling the code symbol puncturing and the symbol repetition. FIG. 3 illustrates a structure in which the channel encoder 312 selects the coding rate under the control of the controller 320.
The coded bits output from the channel encoder 312 are applied to a rate matcher 314, where they are subject to rate matching. Commonly, the rate matching is performed through repetition and/or puncturing on the coded bits, when a transport channel is subject to multiplexing or the output bits of the channel encoder are not identical in number to the symbols transmitted over the air. The coded bits rate-matched by the rate matcher 314 are applied to an interleaver 316, where the rate matched coded bits are interleaved. The interleaving operation is to minimize a possible data loss during transmission. The interleaved coded bits are applied to an M-ary modulator 318, where they are subject to symbol mapping according to a modulation mode or technique of QPSK, 8PSK, 16QAM or 64QAM. The controller 320 controls a coding operation of the channel encoder 312 and a modulation mode of the modulator 318 according to a state of the current radio channel. The HSDPA mobile communication system uses Adaptive Modulation and Coding Scheme (“AMCS”) as the controller 320 in order to selectively use the modulation modes of QPSK, 8PSK, 16QAM and 64QAM according to the radio environment. Though not illustrated in the drawing, the CDMA mobile communication system spreads transmission data with Walsh codes W and orthogonal codes PN, so that a corresponding UE can identify a channel transmitting the data and a Node B transmitting the data.
In the foregoing description of the transmitter, the coded bits are not separately described for the systematic bits and the parity bits. However, the coded bits output from the turbo encoder 312 of the transmitter can be divided into systematic bits and parity bits. Of course, the systematic bits and the parity bits output from the channel encoder 312 have different priorities. In other words, in the case where errors occur in transmission data at a given rate, it is possible to perform better decoding when the errors occur in the parity bits than when the errors occur in the systematic bits. The reason is, as stated above, that the systematic bits are the actual information bits, while the parity bits are the bits added to help the receiver to correct transmission errors during decoding.
Therefore, it is possible to map the interleaved systematic bits and parity bits to the bit positions with higher reliability, the bit positions with medium reliability and the bit positions with lower reliability according to their priorities. Recently, a Symbol Mapping method based on Priority (“SMP”) technique has been proposed for increasing system performance by decreasing probability that errors will occur in the systematic bits having higher priority than the parity bits.
For example, in the case of 16QAM, 4 coded bits are mapped to one symbol before transmission in such a manner that the first two bits are mapped to the bit positions with higher reliability and the last two bits are mapped to the bit positions with lower reliability. In the case of retransmission, the retransmission bits are also transmitted with the same reliability at each transmission. That is, the coded bits initially transmitted through the bit positions with higher reliability are transmitted through the bit positions with higher reliability even at retransmission. Similarly, the coded bits initially transmitted through the bit positions with lower reliability are transmitted through the bit positions with lower reliability even at retransmission. Therefore, there is a high probability that errors will occur in specific bits. In this case, the effect of a coding gain may be reduced, because decoding performance of a turbo decoder is improved when its input bits have a homogeneous Log Likelihood Ratio (“LLR”).
Therefore, a new retransmission technique (Hybrid Automatic Repeat Request (“H-ARQ”)) needs to be introduced to the transmitter and the receiver, considering the fact that decoding performance of the turbo decoder is improved when its input bits have a homogeneous LLR.